Psoralens are in a class of photoreactive natural products which readily penetrate cells and viruses and upon excitation with long wavelength ultraviolet light, form chemical crosslinks between complementary strands of double helical regions. We are utilizing this chemistry to map helical regions in cells and virus particles. We have successfully proven that the hairpin at the replication origin on the fd genome is located at one end of the filamentous phage. We are continuing these studies on other single and double-stranded virus particles.